Power control of point to multipoint physical channels

ABSTRACT

A method and apparatus are used to determine an efficient transmit power for point to multipoint (PtM) transmissions by maintaining a database at a base station which specifies which of a plurality of wireless transmit/receive units (WTRUs) are members of each PtM group. The transmit power of each WTRU&#39;s downlink dedicated channel is adjusted to the minimum required power necessary and the PtM transmit power for each PtM group is set such that the PtM transmit power of a PtM group is equal to the greatest of a WTRU in the PtM group plus a PtM power offset.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of U.S. patent application Ser. No.10/632,776, filed Aug. 1, 2003, now U.S. Pat. No. 7,400,861, whichclaims the benefit of U.S. provisional application No. 60/400,602 filedAug. 1, 2002, which are incorporated by reference as if fully set forth.

FIELD OF INVENTION

The present invention relates to wireless communications. Morespecifically, the present invention relates to power control for pointto multipoint (PtM) services.

BACKGROUND OF THE INVENTION

There is a growing desire to use point to multi-point services inwireless communication systems. In point to multi-point (PtM) services,one service is sent from a single point, such as a base station, tomultiple points, such as multiple wireless transmit/receive units(WTRUs). Examples of point to multi-point services are multimediabroadcasts and multicast services.

In traditional point to point (PtP) services, power control allows forefficient use of radio resources. Power control allows a particularwireless transmit/receive unit (WTRU) to receive the PtP service at adesired quality of service (QoS) and minimize interference to otherWTRUs.

In PtP, such as for the third generation partnership project (3GPP),when the WTRU's dedicated downlink physical channel is power controlled,that WTRU typically determines a target signal to interference ratio(SIR) based on the received block error rate (BLER) of the dedicatedphysical channel. The WTRU estimates the received dedicated physicalchannel's SIR. One approach to determine the SIR is as the ratio ofreceived signal code power (RSCP) over the interference signal codepower (ISCP).

When the WTRU determines that the SIR target value is greater than thecalculated estimate of the received SIR value, the WTRU signals via thetransmit power control (TPC) commands to the base station to increasetransmit power of the downlink dedicated channel. When the SIR targetvalue is less then the received SIR calculated estimate, TPC commandsare generated to decrease DL transmit power.

One channel currently proposed for potentially supporting PtM servicesis the forward access channel (FACH). The FACH is a channel broadcastthroughout a cell and the FACH is maintained at a power level so thatany user in the cell can receive the FACH. As a result, adaptive powercontrol mechanisms are not used for the FACH. One problem with the lackof FACH power control is that a high data rate service sent over theFACH will generate considerable interference. The FACH transmissionpower level needs to be set at a power level so that a WTRU at theperiphery of the cell can receive the high data rate service at anacceptable quality.

Accordingly, it is desirable to have adaptive power control for PtMservices.

SUMMARY

A method and apparatus are used to determine an efficient transmit powerfor point to multipoint (PtM) transmissions by maintaining a database ata base station which specifies which of a plurality of wirelesstransmit/receive units (WTRUs) are members of each PtM group. Thetransmit power of each WTRU's downlink dedicated channel is adjusted tothe minimum required power necessary and the PtM transmit power for eachPtM group is set such that the PtM transmit power of a PtM group isequal to the greatest of a WTRU in the PtM group plus a PtM poweroffset.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a flow chart for power control of a PtM service usingassociated dedicated channels.

FIG. 2 is a simplified diagram of a base station and a WTRU for powercontrol of a PtM service using associated dedicated channels.

FIG. 3 is a flow chart for power control of a PtM service usingassociated dedicated channels.

FIG. 4 is a simplified diagram of a base station and a WTRU for powercontrol of a PtM service without using associated dedicated channels.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Although the preferred embodiments are described in conjunction with athird generation partnership program (3GPP) wideband code divisionmultiple access (W-CDMA) system, the embodiments are applicable to anywireless system using PtM services.

The present invention will be described with reference to the drawingfigures wherein like numerals represent like elements throughout.Hereafter, a wireless transmit/receive unit (WTRU) includes but is notlimited to a user equipment, mobile station, fixed or mobile subscriberunit, pager, or any other type of device capable of operating in awireless environment. When referred to hereafter, a base stationincludes but is not limited to a base station, Node-B, site controller,access point or other interfacing device in a wireless environment.

The present invention is described subsequently for three differentgeneral implementations. In a first implementation, each WTRU receivingthe PtM service has associated dedicated channels for use in supportingthe PtM service. In a second implementation, the WTRUs receiving the PtMservice do not have dedicated channels for use in supporting theservice. In a third implementation, some of the users have dedicatedchannels for use in supporting the service and others do not.

FIG. 1 is a flow chart for adaptive power control for a PtM service whenassociated dedicated channels are available. FIG. 2 is a simplifiedblock diagram of a base station 54 and WTRU 56 for sending and receivingsuch a service. The PtM service data may be sent over one of variouschannels, such as a shared channel, high speed shared channel asproposed for W-CDMA or a common channel. For the PtM service, multipleWTRUs 56 registered for the service receive that service over the PtMchannel simultaneously.

For each WTRU 56 that enters the PtM service area and is registered forthat service, an uplink and a downlink dedicated physical channels areestablished, step 20. The dedicated physical channel may be independentor comprised of separate dedicated physical channels for control anddata, or just physical control channels.

As shown in FIG. 2 for the downlink (DL) dedicated channel associatedwith the PtM channel, a DL dedicated channel transmitter 30 produces thechannel. An amplifier adjusts the transmission power level of the DLdedicated channel and an antenna 42 or antenna array radiates the DLdedicated channel through the wireless interface 44. At the WTRU 56, aDL dedicated channel receiver 50 coupled to the WTRU antenna 46 receivesthe channel.

Each WTRU 56 estimates a reception quality of the DL dedicated channel,such as a received signal to interference ratio (SIR), step 22. The SIRmay be measured using the received signal code power (RSCP) andinterference signal code power (ISCP) associated with the DL dedicatedphysical channels. The estimated reception quality is compared with atarget reception quality, such as a target SIR. Based on the comparison,transmit power control (TPC) commands are generated by a TPC commandgenerator 52. The TPC commands are sent to the base station 54, such asusing the uplink dedicated channel or as a layer 3 message on a commonuplink channel.

A TPC receiver 40 at the base station 54 receives the commands. The TPCcommands are used to adjust the transmit power of the DL dedicatedchannel to achieve the target reception levels, such as target SIR andblock error rate (BLER) requirements, for the quality of service (QoS)desired. The power amplifier 34 of the DL dedicated channel is changedaccordingly.

For each power controlled PtM physical channel or set of physicalchannels, the base station equipment maintains a database of whichspecific WTRUs 56 receive which particular PtM channels. The group ofWTRUs 56 associated with each PtM channel is referred to as a PtM Group(PtM-G). A WTRU 56 can be member of more than one PtM-G.

The transmit power of each WTRU's DL dedicated channel or set ofdedicated channels is adjusted to the minimum required power to theminimum required power necessary to achieve the respective QoSrequirement for that WTRU 56. Preferably, for each WTRU 56, the transmitpower of the PtM physical channel or set of physical channels is derivedfrom the current transmit powers of the associated DL dedicated channelswithin the PtM-G, step 26. One approach to determine the required PtMchannel power for a WTRU 56 of the PtM-G is according to Equation 1 orEquation 2.PtM _(—) TxPwr=DL _(—) DchPwr+PtM_Power_Offset  Equation 1PtM _(—) TxPwr=DL _(—) DchPwr*PtM_Power_Ratio  Equation 2

PtM_TxPwr is the desired transmit power of the PtM channel for that WTRU56. DL_DchPwr is the transmission power of that WTRU's DL dedicatedchannel or channels, adjusted according to TPC commands and theconfigured TPC step size. PtM_Power_Offset is an adjustment to correctfor differences between the DL dedicated channel and the PtM channel,such as coding rate, QoS, etc. PtM_Power_Ratio is a ratio to correct fordifferences between the DL dedicated channel and the PtM channel.

The PtM power offset and the PtM power ratio are, preferably, derivedusing multiple factors as illustrated in Equation 3 for the PtM poweroffset and Equation 4 for the PtM_Power_Ratio.PtM_Power_Offset=RelDch+RelTF+RelQoS+X  Equation 3PtM_Power_Ratio=RelDch*RelTF*RelQos*X  Equation 4RelDch is a factor configured by the operator to correct between thepower offset between the dedicated channel and the PtM channel. RelTF isa factor to compensate for the difference in transport data block setsize and coding rate between the dedicated and the PtM channel. RelQoSis a factor to compensate between the BLER requirements between thededicated and the PtM channel. X is a general factor for any otherrelative transmit power offsets/ratios which may be applied.

The PtM transmit power (PtM_Tx_Pwr_PtM-G) is calculated by determiningthe maximum WTRU PtM transmit power requirement within the PtM-G perEquation 6.PtM _(—) Tx _(—) Pwr _(—) PtM-G=MAX(PtM _(—) TxPwr(WTRU))  Equation 6PtM_TxPwr(WTRU) is the set of determined PtM transmission power levels,PtM_TxPwr, for each user of the group, group G. MAX(PtM_TxPwr(WTRU)) isthe maximum PtM transmission power level out of this group. By using themaximum PtM transmission power level required by any WTRU 56 in thegroup, it ensures that all the other WTRUs 56 in the group (whichrequire less PtM transmit power) will be able to receive the PtM signal,step 28. The PtM transmit power may be recalculated and adjusted on aslot, radio frame, or transmission time interval (TTI) basis, amongother time periods, for optimal performance.

A PtM transmitter (Xmitter) 32 produces the PtM channel. A transmitpower calculation device 38 adjusts the transmit power of the PtMchannel, such as by changing a gain of a power amplifier 36, to thedesired transmission power level. The base station's transmission powerlevel is adjusted in accordance with the highest WTRU transmission powerrequirement. The TPC commands from all the WTRUs 56 in the group areprocessed to determine the power adjustment. Essentially, to increasethe transmission power of the PtM requires only a single WTRU 56 torequest an increase in transmission power. For the transmission power todecrease, all the WTRUs 56 in the group need to request a decrease inpower.

Equation 7 is one possible equation for determining the poweradjustments for the PtM transmission.New_(—) PtM_Power_(—) PtM-G=Current _(—) PtM_Power_(—)PtM-G+Ptpc+Pbal  Equation 7Current_PtM_Power_PtM-G is the current PtM transmit power. Ptpc iseither an increase or decrease by a step size. The Ptpc adjustment ispreferably a configured power control step size (0.5, 1, 1.5 or 2 dB),which either increases or decreases the transmission power level basedon the received TPC commands. Pbal is an optional correction forbalancing towards a common reference power.

FIG. 3 is a flow chart for adaptive power control for a PtM service whendedicated channels are not available or are not used to support the PtMservice. FIG. 4 is a simplified block diagram of a base station 54 andWTRU 56 for sending and receiving such a service.

A PtM transmitter (Xmitter) produces a PtM channel. The transmissionpower level of the PtM channel is controlled, such as by an amplifier36. The initial PtM transmission power level may be a power levelpreconfigured by an operator that allows for full cell coverage or bebased on RSCP and ISCP measurements of WTRUs 56 in the PtM group. ThePtM channel is radiated by an antenna 42 or antenna array of the basestation 54, though a wireless interface 44. The PtM channel is receivedby an antenna 46 of each WTRU 56 associated with the PtM service. A PtMreceiver recovers data from the PtM channel.

A TPC command generator sends TPC commands to the base station 54 forthe PtM. The TPC commands may be based on the SIR of the received PtMchannel or another channel received by the WTRU 56, such as a channelreceived by multiple ones of the WTRUs 56 in the group, steps 58 and 60.The SIR may be derived using the RSCP and ISCP values, pathloss and/orBLER of the measured channel.

One preferred technique for getting these measurements is to usephysical control signaling. The measurements, such as RSCP, ISCP and/orpathloss, are signaled directly in physical control signaling or withinL2 header information of uplink common channel transmissions. Thisprocedure is similar to the procedure that the initial power of the PtMchannel is set. The measurements updates would, preferably, be providedon a “best effort” basis, depending on the availability of the uplinkchannels to the WTRUs. For example, a “persistency” indication fortransmission and “access service class” partitioning of the uplinkcommon channel may be used.

Equation 8 is one possible equation for use with this common channel,for calculating the PtM transmission power, PtM_TxPwr.PtM _(—) TxPwr=DL _(—) PtM _(—) Pwr*a*(TargetRSCP/ISCP)/(Measured(RSCP/ISCP)  Equation 8DL_PtM_Pwr is the previous PtM transmission power setting. “a” is anoperator controlling factor effecting the RSCP/ISCP ratio. Alternately,the pathloss may replace the RSCP/ISCP ratio in Equation 8.

A TPC receiver at the base station 54 receives the TPC commands, step62. Using the received TPC commands, a transmit power calculation deviceadjusts the transmit power level of the base station 54. The basestation's transmission power level is adjusted in accordance with thehighest WTRU transmission power requirement. The TPC commands from allthe WTRUs 56 in the group are processed to determine the poweradjustment. Essentially, to increase the transmission power of the PtMrequires only a single WTRU 56 to request an increase in transmissionpower. For the transmission power to decrease, all the WTRUs 56 in thegroup need to request a decrease in power, step 64.

In another implementation, some of the WTRUs 56 have dedicated channelsfor use in power control of the PtM channel and others may not. In suchan implementation, power control can be performed without using thededicated channels such as per FIGS. 3 and 4. However, preferably, WTRUs56 having dedicated channels use those channels to generate TPC commandand the WTRUs 56 not having dedicated channels use other channels, suchas the PtM channel or a channel common to multiple WTRUs 56 in the groupto generate the TPC commands. The base station 54 sets its transmissionpower level based on the commands from all the WTRUs in a particular PtMgroup. Essentially, to increase the transmission power of the PtMrequires only a single WTRU 56 to request an increase in transmissionpower. For the transmission power to decrease, all the WTRUs 56 in thegroup need to request a decrease in power.

1. A method of determining an efficient transmit power for point tomultipoint (PtM) transmissions, the method comprising: maintaining adatabase at a base station (BS) which specifies which of a plurality ofwireless transmit/receive units (WTRUs) are members of each PtM group;adjusting a transmit power of each WTRU's down link (DL) dedicatedchannel to the minimum required power necessary to maintain DLcommunications with each WTRU; and setting a PtM transmit power for eachPtM group, wherein the PtM transmit power of a PtM group is equal to thegreatest of a WTRU in the PtM group plus a PtM power offset.
 2. Themethod of claim 1 wherein the PtM power offset corrects for differencesbetween the DL dedicated channel and the PtM channel.
 3. The method ofclaim 1 wherein the PtM power offset is based on multiple factorsincluding a coding rate.
 4. The method of claim 1 wherein the PtM poweroffset is based on multiple factors including a quality of signal. 5.The method of claim 1 wherein the PtM power offset is a ratio to correctfor differences between the DL dedicated channel and the PtM channel. 6.The method of claim 5 wherein the ratio is based on multiple factorsincluding a difference in transport data block set size and coding ratebetween the dedicated channel and the PtM channel.
 7. The method ofclaim 5 wherein the ratio is based on multiple factors including afactor to compensate between the block error rate (BLER) requirementsbetween the dedicated and the PtM channel.
 8. The method of claim 1wherein the minimum required power necessary to maintain DLcommunications is equal a minimum required power necessary to achievethe QoS requirement of the WTRU.
 9. A base station configured fordetermining an efficient transmit power for point to multipoint (PtM)transmissions comprising: a processor configured to: specify which WTRUsare members of which PtM groups; determine a minimum transmit power ofeach the WTRU's down link (DL) dedicated channel; and set a PtM transmitpower for each PtM group, wherein the PtM transmit power of a PTM groupis equal to the greatest of a WTRU in the PtM group plus a PtM poweroffset.
 10. The base station of claim 9 wherein the PtM power offsetcorrects for differences between the DL dedicated channel and the PtMchannel.
 11. The base station of claim 9 wherein the PtM power offset isbased on multiple factors including a coding rate.
 12. The base stationof claim 9 wherein the PtM power offset is based on multiple factorsincluding a quality of signal.
 13. The base station of claim 9 whereinthe PtM power offset is a ratio to correct for differences between theDL dedicated channel and the PtM channel.
 14. The base station of claim13 wherein the ratio is based on multiple factors including a differencein transport data block set size and coding rate between the dedicatedchannel and the PtM channel.
 15. The base station of claim 13 whereinthe ratio is based on multiple factors including a factor to compensatebetween the block error rate (BLER) requirements between the dedicatedand the PtM channel.
 16. The base station of claim 9 wherein the minimumrequired power necessary to maintain DL communications is equal aminimum required power necessary to achieve the QoS requirement of theWTRU.